How to stop onload play html5 video on websites?

Anyone know how stop automatic play on page load unwanted html5 videos? This is importnant for me because I use limited bandwith transfer, and that unwanted media eat my amount of limited transfer. Thank you in advance for help.

Solution 1:

• Enter about:config into Firefox’s address bar and press Enter.
• Agree to the warning.
• Type autoplay into the search box.
• You’ll see a preference named media.autoplay.enabled, which will be set to True.
• Double-click that preference and it will change to False.

Be warned that animated GIF loads normally, but doesn't play. Many times you're not aware of this, and just see an image and think that's it. This happens to me on Reddit a lot. Then you have to right-click the image and select Play.